4/4 is a time signature. The most commonly used one, and probably the only one you'll be using in rap. Odd time signatures are more common in prog rock, jazz, metal ect
If you listen to Pink Floyd's "Money" you can hear its a weird time signature, its 7/4 for most of it, switching to 4/4 for the guitar solo.
http://en.wikipedia.org/wiki/Time_signature
BPM = beats per minute. IE how fast the song is.
http://www.metronomeonline.com/
So time signatures pertain to the timing of the song and BPM to the tempo.